Distortions of spatial perception index perceptual organization

The presence of objects in a scene distorts perceived spatial relationships. For example, two dots are perceived as farther apart when they are presented well within an object's boundaries, compared to an equivalently spaced pair of dots that are placed in a ground region. A strong, opposing compressive effect on perceived space occurs if the dots are placed near, on, or slightly beyond opposite-side edges. Both expansion and compression effects generate compelling illusions. In the present study, we examined whether the degree of compressive warping indexes the amount of organization present in a region of space. Subjects viewed scenes composed of a dense grid of Gabor grating patches, with one pair of red, vertically-aligned "reference" dots that never changed spacing, and one pair of "adjustment" dots whose spacing varied trial-to-trial according to a staircase procedure. Outside of one rectangular region (the "object region"), gratings were oriented randomly. The object region either surrounded (expansion-spacing) or was flanked by (compression-spacing) the reference dots. Within the object region, we varied the "coherence" of the gratings by fixing a proportion of gratings to the same horizontal orientation, altering the clarity of the region's outline: baseline (0%, all random), low (50% horizontal gratings), medium (75% horizontal), and high (100% horizontal). In high-coherence scenes, grouping and contour formation produced a percept of a rectangular "object." Using a 2AFC method with a staircase procedure (N=45), we determined the point of subjective equivalence for each combination of reference spacing and coherence. Strong expansion and compression effects occur for the high-coherence display vs. baseline. Critically, both expansion and compression effects were titrated by coherence, with weaker effects corresponding to lower coherence. We conclude that perceptual organization distorts perceived spatial relationships, in a manner that can be used to quantitatively assess the degree of organization in a visual region.